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Effect of Light on Some Reactions


Light, being a source of energy can affect the rate of some chemical reactions by considerably energizing some of the molecules involved. For example, in the chain reaction between hydrogen and chlorine.

The process of the reaction is thus: in the presence of light, chlorine molecule is energized and breaks up into atoms (usually called free radicals), The free radicals, being very reactive then combine rapidly with hydrogen molecules to form hydrochloric acid.

The influence of light on the reaction is evident if the reaction is done at ordinary pressure in (i). Darkness (ii). Daylight and (iii). Sunlight.

In darkness, the reaction is negligible (i.e., almost not occurring); in daylight, it is slow; while in sunlight, it is explosive. The same effect is observed of light on the reaction between alkanes, example, methane and the halogens, such as chlorine.

The reaction also follows the same process as the reaction between hydrogen and chlorine above. Note: comparing the reactivity of the halogens, we have F>Cl > Br > I.

This means that fluorine will combine with hydrogen or methane faster and greater in the presence of light than chlorine - infact, the reaction of fluorine is explosive - this is why it is not usually used in experiments to study the halogens. Iodine will react least.

Other reactions which light affect are: the photosynthetic production of starch by plants from carbondioxide and water; the decomposition of silver halides (these substances are very sensitive to light). The decomposition of silver halides by light - silver halides are silver compounds of the halogens. I.e., AgCl , AgBr and AgI.

When these substances are exposed to direct sunlight, they decompose completely to give a grayish substance (metallic silver) and emit electrons. However, in a dark cupboard they remain unchanged (i.e., no decomposition).

Their decomposition is dependent on the degree of light they are exposed to.

Note: comparing the light sensitivities of the silver halides, we have: AgBr > AgCl > AgI.

Due to the highest sensitivity of AgBr to light, it is used as the main compound suspended in the gelatin of a photographic plate.  

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